Engines in which a piston reciprocates in a cylinder generate primary inertial force resulting from the reciprocative movement of the piston. The primary inertial force causes engine vibrations. The engines of this type include the so-called V-type engine in which cylinders are arranged in a V-shape. If an angle of the cylinders in the V-type engine is 90 degrees, the primary inertial forces resulting from the reciprocative movement of the pistons in the cylinders cancel each other to reduce engine vibration.
On the other hand, if the angle of the cylinders is smaller or greater than 90 degrees, the primary inertial force remains unchanged. To avoid this, a motorcycle engine comprising a primary balancer adapted to cancel the primary inertial force is provided (for example, see Patent Documents 1 and 2).    Patent Document 1: JP-A No. 2005-16511    Patent Document 2: JP-P No. Hei 2-49422